Gases employed in the semiconductor industries are required to have high purity levels, and also sensitive monitoring and analytical techniques, not of the laboratory level, but of the level employable on the spot, have been strongly desired. Meanwhile, gases are required to have guaranteed levels of as high as from the sub ppm levels to the 10-1 digit ppb levels, and the conventional gas chromatographic analyses are being replaced by analyses using sensitive mass spectrometers or gas chromatograph mass spectrometers. Particularly, among the mass spectrometers, there are recently an increasing number of atmospheric pressure ionization mass spectrometers which carry out ionization under atmospheric pressure to enable sensitive determination.
However, since mass spectrometers and gas chromatograph mass spectrometers are expensive, installation of a plurality of such analyzers to cope with a plurality of gases, respectively, for continuous monitoring results in tremendous costs. Further, since installation of a plurality of analyzers is not preferred either on the standpoint of space saving, a multi-gas analysis system which is directed to analyses of a plurality of gases using one analyzer is desired.
To describe the multi-gas analysis system, the system has one main analyzer and a gas selector installed on the upstream side of the analyzer. Thus, a gas is selected and introduced to the analyzer, and after completion of a determination of the gas, valves are changed over to select another gas species to be analyzed for determination. Although the gases cannot be analyzed continuously due to change-over operations, if the change-over operation is repeated at short intervals, the gases can be analyzed substantially continuously. Consequently, a plurality of gas species can be analyzed by a single analyzer main body, leading to a reduction in the facility cost and in the analyzer installation area.
As described above, although the multi-gas analysis system enjoys many merits, contamination to be caused by mixing of gases occurs easily compared with the case where analyzers are installed for the analyses of the gases respectively, since various kinds of gases are allowed to flow through the gas selector installed on the upstream side of the analyzer. Particularly, since change-over operation is carried out frequently for a plurality of sample gases at desired intervals, external leakage due to damage or abrasion of valves employed or internal leakage due to damage of valve seats can occur easily to be causative of contamination. For example, depending on the kind of gas causing the internal leakage, measured values of impurity contents in the gas actually in determination are affected by the contamination like external leakage to be a hindrance to accurate analysis. External leakage also causes not only migration of air to contaminate the gas in the piping, but also errors in the analytical values.
However, what should be noted here is that such internal leakage and external leakage should be avoided absolutely for security sake, since they can cause accidents if they occur when highly reactive gases are being analyzed. For example, for the external leakage, security can be maintained by detecting hydrogen as a combustible gas and oxygen using a hydrogen detector, etc. and an oxygen detector, respectively, and by detecting other incombustible gases (nitrogen, argon, helium, etc.) if leaked in large amounts using an oxygen deficiency meter.
However, in the case where hydrogen and oxygen used in electronic industries whose purity levels should be monitored strictly are to be analyzed using the multi-gas analysis system as described above, occurrence of an internal leakage due to damage of a valve seat and the like when the gas is changed over to the other, these two gases are mixed in the system to be liable to cause fires, explosions, etc. In order to avoid such combination of the gases, separate analysis systems should be installed for hydrogen and oxygen respectively. Meanwhile, occurrence of an external leakage in a passage of silane, arsine, etc. which are semiconductor material gases can also cause explosion, so that such leakages should be detected and coped with as soon as possible. However, in the commercially available leakage detectors, there is no suitable one which is satisfactory in response and sensitivity, and which can be utilized as inserted to the gas passage.
It is an objective of the present invention to provide an apparatus and a method for analyzing gases, which ensure detection of presence and absence of leakage in a short time and which enable analyses successively, safely and accurately in a stable state.